A power cut or other event may cause a large number of SIP devices (i.e. devices which use SIP, such as VoIP phones) in a network to be disconnected from a central network resource such as a server running a SIP registration application (“registrar”). Following the power cut, each of those devices may attempt to send one or more messages of a similar kind to the central resource. For example, once power has been restored, some or all of the SIP devices in the network may send a SIP “REGISTER” message towards the registrar in the network, at approximately the same time, to begin SIP registration processes for the devices. The large number of messages sent towards the registrar at approximately the same time may be referred to as a “storm” of messages.
Depending on the number of SIP devices and the resources available, the registrar might not be able to process all of the incoming REGISTER messages. This might lead to the registrar becoming overloaded and crashing, and/or to one or more of the incoming REGISTER messages not being responded to. In such circumstances, the SIP devices may re-transmit their respective REGISTER messages, exacerbating the problem by generating yet more messages.
To try to prevent crashing of the registrar, an intervening network element, such as a session border controller (“SBC”), might prevent or delay one or more of the REGISTER messages from reaching the registrar. The SBC may, for instance, forward only a certain number of register messages to the registrar in a period of time. This may mean that only 10% (for example) of the REGISTER messages sent by the SIP devices reach the registrar. If the SBC is correctly configured, the number of REGISTER messages that it allows through should be sufficiently small that the registrar is able to process them all without crashing.
SIP registration (like other SIP processes) can involve authentication of the device seeking to register. This may mean that a device sends a first SIP REGISTER message towards the registrar, and then a second REGISTER message in response to an authentication “challenge” message from the registrar. The “challenge” from the registrar may comprise a “401” message, or a similar kind of SIP response, including authentication information for the device to use in future. The second REGISTER message sent by the device uses the authentication information from the challenge message to authenticate the device at the registrar. This may prevent unauthorized devices from completing a registration process, but means that every device has to send two REGISTER messages rather than one to complete a registration process.
The second REGISTER message from the device must reach the registrar in order for registration of the device to be complete. If the first REGISTER from the device is allowed through by the SBC, but the second REGISTER from the device is not allowed through by the SBC, the registration process will fail, reducing the number of successful registrations. Thus, if the SBC cannot discriminate between first REGISTER messages and second REGISTER messages, it may be that, in the example discussed above, the SBC allows through only 10% of second REGISTER messages, leading to successful registration of only 1% (i.e. 10% of 10%) of devices requesting registration. This may lead to unacceptable delays in registration of SIP devices, and to unnecessarily high numbers of packets being sent over a network, as some devices re-send their second REGISTER messages one or more times to complete their respective registration processes.
The invention aims to go at least some way towards solving the problems identified above.